The present invention relates to the bonding of two dissimilar polymeric materials, and more particularly to the bonding of a solid rocket propellant to a silicone rubber liner.
Silicone rubber, as used in this specification, encompasses two major product types, heat cured rubber and room temperature vulcanizing rubber, (RTV), whose compositions and properties are well known to those skilled in the art.
It is often difficult to get one layer of polymeric material to bond to another, especially where the chemical natures of the two layers are very different. The layers may have different surface energies, they may not wet each other well, or there may be steric hindrance between the molecules of the two layers.
This bonding problem has been found to be especially acute in applications of silicone rubber. In fact, silicone rubber is often used as a mold for casting plastics because of its inertness and quick release characteristics toward most plastics, e.g., vinyl plastisols, epoxies, polyester resins, and polyurethanes.
The bonding problem occurs in the field of rocketry because silicone rubber is used as a liner between the metal casing of the rocket and the solid propellant fuel grain. It insulates the casing from the propellant grain both thermally and mechanically. The overall performance characteristics of the rocket are highly dependent on the strength of the bond between the propellant, the liner, and the casing.
It is known in rocket technology to chemically crosslink the liner to the propellant grain. Webb, in U.S. Pat. No. 3,507,114, and Skidmore in U.S. Pat. No. 3,813,308 both describe such chemical bonding techniques. Webb describes the bonding of a polybutadiene propellant to a HTPB (hydroxy terminated polybutadiene) liner by adding to the liner an excess of diisocyanate crosslinking agent. Skidmore describes the bonding of a carboxy-containing rubber liner, such as butadiene, to a rubber insulator, preferably styrene butadiene, by coating the surface of the insulator with a polyisocyanate, preferably a di- or tri-isocyanate, thereby crosslinking the liner to the insulator.
Due to the above-mentioned incompatability of silicone rubber with butadiene rubber etc., bonding techniques such as Webb's and Skidmore's cannot be used with silicone rubber liners.